A known internal combustion engine includes an exhaust-gas recirculation device (EGR) for recirculating a part of exhaust gas of the internal combustion engine to an intake channel. In such an internal combustion engine, especially in a diesel engine, when a change in the driving state increases an EGR circulation amount, more smoke (PM) may be produced due to an insufficient intake flow rate. Further, a decrease in the EGR circulation amount may lead to loss of the NOx reduction effect. In view of this, to reduce the discharge amount of PM and NOx contained in exhaust gas, it is required to control the EGR circulation amount suitably in accordance with driving conditions.
Patent Documents 1 and 2 disclose an example of a control for an EGR device of such type.
Patent Document 1 discloses controlling the opening degree of an EGR valve by calculating an air-excess ratio in a cylinder from a fuel-injection amount, an intake-air amount, and a non-combusted air amount in EGR gas returned to an intake system, and obtaining an opening-degree target value for the EGR valve on the basis of the air-excess ratio.
Patent Document 2 discloses controlling the opening degree of an EGR valve using a control parameter which is an EGR rate calculated as a ratio of exhaust gas to intake air of an internal combustion engine on the basis of a detection value of a temperature sensor.